How many protons does 123Sb have?

Answer 1

51, just like all the other isotopes of Antimony.

The amount of protons does not change in any of the isotopes of any element.

If it were to it would be a different element.

Answer 2

Antimony (Sb) is element number 51, which means it has 51 protons. So, 123Sb specifically would also have 51 protons.